Demonstration 2: Colloids, The Tyndall Effect and More
Tyndall effect is usually given as a definitive test to distinguish between
a true solution and a colloid. The Tyndall effect involves the scattering
of a beam
of light as the light passes through a medium having particles of colloidal size. Since particles such as molecules of sugar or sodium ions or chloride ions in
solution are too small to scatter light, a beam of light passing through such a solution is not scattered. However, the protein molecules in milk are of colloidal
size and consequently a drop of milk mixed into water will cause a light beam traversing the solution to be scattered. To demonstrate the difference, the two
systems described above are usually employed. The milk in water changes color as more milk is added (bluish to yellow to red). However, a single system
where the particles go from “solution size” to “colloidal size” provides a more dramatic demonstration and as the articles continue to grow additional optical
effects may be demonstrated. One such system is the production of sulfur by the reaction between sodium thiosulfate and sulfuric acid.
Na 2 S 2 O 3 (aq) + H 2 SO
(aq) ® S(s) + SO 2 (g) + H 2 O(l) + Na 2
SO 4 (aq)
the particles of sulfur grow from solution size to colloidal size and finally
begin to precipitate. As this phenomenon occurs, it is possible to demonstrate
the Tyndall effect and to examine some of the characteristics of the scattered and transmitted light. These characteristics may then be related to other
phenomena, such as red sunsets.
students are very allergic to sulfur dioxide (SO 2 ), which
is generated during this demonstration. You should offer a warning and
excuse students if
they have a known sensitivity to sulfur dioxide (or to sulfites in food, which may signal such a sensitivity).
Square or rectangular battery jar or a small fish tank
Stirring rod (long glass rod to stir contents of the battery jar)
Parallel beam light source or a flashlight
Piece of frosted glass or a sheet of white paper stapled to a frame
Ring stand and clamp to hold the glass or paper in a vertical position
Preweighed sample of Na 2 S 2 O 2 or Na 2 S 2 O 3 ·5H 2 O to make a 0.01 m solution when dissolved in the water in the battery jar (1.6 g Na 2
S 2 O 3 per L water or 2.5 g Na 2 S 2 O 3 ·5H 2 O per L water) 10 mL Concentrated sulfuric acid, H 2 SO 4 per L of water in the battery jar
One or more pieces (sheets) Polaroid material
Cardboard silhouette of a flying duck (about the size of half the diameter of the parallel beam) and hung like a mobile from a fine piece of string
Recording of “Canadian Sunset” and appropriate player.
Dispose of the battery jar solution as soon as the demonstration is ended to minimize the amount of SO 2 entering the atmosphere. After summarizing
The demonstration should be conducted in a well ventilated area because of the production of SO 2 (g) whose odor can be mildly detected while the demonstration is in progress.
Set up the light source, battery jar, and frosted glass as illustrated in Figure 2. The battery jar should be filled with sufficient depth of water so that the entire diameter of the light beam passes through the liquid. (The amount of water should be predetermined in order to have a preweighed sample of sodium thiosulfate and a premeasured volume of concentrated sulfuric acid prepared.) Also, the water should be placed into the battery jar a few minutes prior to beginning the demonstration to permit air bubbles to escape.
Turn on the light source and darken the room. If you will be using the Polaroid sheets, pass these out to students seated directly in front of the demonstration. Ask students whether they can see the beam traversing thewater. If using the Polaroid sheets, ask whether rotating them has any effect. [They should not be able to see the beam but the white round disk of the transmitted beam hitting the frosted glass should be visible. Nothing different should be seen using the Polaroid sheet.] Point out that the transmitted beam striking the frosted glass is colorless (white). Add the preweighed sample of sodium thiosulfate to the water and stir until it is dissolved. Wait for any bubbles to leave the solution. Repeat the above question(s). [The reply should be the same.] Carefully and with stirring, add the premeasured concentrated sulfuric acid to the sodium thiosulfate solution. Wait about 15 seconds for bubbles to clear and repeat the above question(s). [The reply should be the same.] Ask if there is any evidence of a chemical reaction? [There should be no such evidence.] Ask students to tell you when they notice any change. [Using the concentrations recommended and a solution temperature of about 20 °C, it should take about two minutes for the Tyndall beam to start to appear and another two minutes for the particles to become large enough and concentrated enough for the percent of transmitted light to go to zero. The reaction rate appears to be first order in thiosulfate concentration so if you want to slow things down, decrease the concentration of sodium thiosulfate.] As soon as the Tyndall beam becomes visible ask students to compare the color of the scattered light to that of the transmitted light striking the screen. Also ask students using the Polaroid sheets whether rotating them has any effect on the intensity of the scattered light. [The scattered beam is bluish, whereas the transmitted light starts to turn yellow. As the Tyndall beam becomes more visible the scattered beam loses a little of its bluish color as the green and yellow portion of the spectrum is scattered and the transmitted
light striking the screen goes from yellow to red. This observation occurs because the wavelengths of the light scattered are directly proportional to the size of the particle scattering the light. Also, the scattered light is polarized.] If you have the silhouette of a “flying duck” you can add a dramatic closing touch to the demonstration. Start the recording of “Canadian Sunset” and hang the silhouette over the frosted glass so that it intercepts the disk of the reddening transmitted beam before the beam strikes the glass.
T O P V I E W
You may follow the demonstration with the following questions:
1. Why does the sun appear exceptionally red when it sets behind a city?
[The blue end of the visible
spectrum is scattered by the dust and aerosols in the air over the city and the red end of the spectrum is
2. Why are fog lights usually yellow and not white? [The yellow light being of relatively long wavelength is
transmitted through the colloidal size fog particles thereby permitting the driver to see, whereas the blue
component of white light is scattered back to the driver thereby obscuring vision.]
3. Why not use red fog lights? [Red light is usually a signal of danger and approaching motorists may interpret
it as such. Also, the human eye is much more sensitive to yellow light than red light.]
4. Why do things look clearer through rose (pink) colored glasses? [The pink glass filters out blue light that is
scattered by aerosols in the environment. Since this blue light does not reach our eyes, the clarity of objects
Beaker of water
Milk, 10 mL
A simpler and quicker demonstration involves a beaker of water on the stage
of an overhead projector and a small amount of milk in a medicine dropper.
As the milk is slowly added to the water, the color of the water becomes bluish, while the circle of light thrown up onto the screen gradually becomes
yellow then orange and then red. The questions suggested above (Option A) are also appropriate for this option.
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